Twelve young adults completed a 14-day inpatient crossover study with five evenings of eReader reading and five evenings of printed reading, each for four hours before bedtime.

Compared with printed reading, the eReader condition suppressed evening melatonin by about 55%, delayed melatonin onset by about 1.5 hours, lengthened time to fall asleep, and reduced REM (the dream-heavy sleep stage) sleep.

Seventy-two healthy men were exposed to display light for four hours before bedtime under different melanopic settings that change how strongly the light stimulates melanopsin, the blue-light-sensitive receptor that shifts circadian timing.

Higher melanopic exposure produced a dose-related increase in sleep latency and greater melatonin suppression, while low-melanopic settings led to earlier melatonin timing and faster sleep onset.

Participants used a computer screen for five hours in the evening under either a white LED-backlit screen or a non-LED screen, which differed mainly in their blue-light output.

The LED-backlit screen suppressed the normal evening rise in melatonin and reduced objective and subjective sleepiness measures such as slow eye movements and low-frequency brain activity.

Twenty-two adults completed six evening light conditions with different spectral compositions that varied the intensity of melanopsin-stimulating blue light while holding overall brightness similar.

The timing of the melatonin rise, subjective sleepiness, and EEG (brain-wave) measured sleep onset shifted across light spectra, showing that blue-heavy light delays sleep timing even when overall brightness is controlled.

Eight healthy men were exposed to two hours of narrow-band blue light (460 nm), green light (550 nm), or darkness before sleep on different nights with equal photon exposure.

Blue light shortened REM sleep in the early sleep cycles and altered slow-wave brain activity, indicating that short-wavelength light changes sleep architecture.

Eight healthy adults were exposed to increasing irradiances of narrow-band blue LED light between 2:00 and 3:30 AM.

Melatonin was suppressed in a dose-dependent way as blue-light intensity increased, showing that short-wavelength LED light can strongly blunt the sleep hormone at night.

Eleven men and eight women completed night-time testing under three conditions: bright light, bright light filtered below 530 nm, and dim light.

Under unfiltered bright light, melatonin was strongly suppressed, while the filtered condition preserved melatonin to levels similar to dim light without worsening performance, indicating that short-wavelength light drives melatonin suppression.

Fourteen adults with insomnia symptoms completed two 7-day periods wearing amber lenses and clear lenses in the two hours before bedtime.

Total sleep time increased from about 347 minutes with clear lenses to about 399 minutes with amber lenses, and sleep-quality ratings improved. The objective sleep measure was actigraphy, a wrist-movement tracker commonly used for sleep timing.

Twenty adults wore amber or yellow-tinted glasses for three hours before bedtime over a two-week period.

The amber group reported better sleep quality and higher positive mood scores than the yellow-lens group, suggesting that reducing blue light exposure before bed can improve perceived sleep.

Researchers studied medical students ages 20 to 22 and tested whether reducing blue light from smartphone screens at night would improve sleep.

Sleep quality was measured with the Pittsburgh Sleep Quality Index (PSQI). The average PSQI score fell from 6.83 to 3.93 after the blue-light reduction, moving from the "poor sleep" range into the "good sleep" range.

A systematic review screened 128 studies on light exposure and circadian rhythm and identified 15 higher-quality studies that measured melatonin and had at least 20 participants.

Across those studies, two hours of evening blue light around 460 nm suppressed melatonin, with the largest effects at shorter wavelengths. Melatonin is a circadian signal for nighttime, so suppressing it indicates a biological shift that can delay sleep timing.